Radio Frequency Identity tags or RFID tags have a long history and have in recent times RFID has become synonymous with “passive backscattered transponders”. Passive transponders obtain power and a clock reference via a carrier and communicate by detuning an antenna, often with a fixed pre-programmed ID. These tags are designed to replace barcodes and are capable of low-power two-way communications. Much of the patent literature surrounding these radio tags and RFID tags as well as the published literature uses terminology that has not been well defined and can be confusing. We provide a glossary of words and concepts as used within this document:                Radio Tag—any telemetry system that communicates via magnetic (inductive communications) or electric radio communications, to a base station or reader or to another radio tag.        Passive Radio Tag—A radio tag that does not contain a battery.        Active Radio Tag—A radio tag that does contain a battery.        Transponder—A radio tag that requires a carrier wave from an integrator or base station to activate transmission or other function. The carrier is typically used to provide both power and a time-base clock, only typically at high frequencies.        Non-Radiating Transponder—A radio tag that may be active or passive and communicates via de-tuning or changing the tuned circuit of an antenna or coil. Does not induce power into a transmitting antenna or coil.        Radiating Transponder—A radio tag or transponder that may be an active or passive tag, but communicates to the base station or interrogator by transmitting a radiated detectable electromagnetic signal by way of an antenna. The radio tag induces power into an antenna for its data transmission.        Back-Scattered Transponder—Synonymous with “Non-Radiating Transponder”. Communicates by de-tuning an antenna and does not induce or radiate power in the antenna.        Transceiver—A radiating radio tag that actively receives digital data and actively transmits data by providing power to an antenna. May be active or passive.        Passive Transceiver—A radiating radio tag that actively receives digital data and actively transmits digital data by providing power to an antenna, but does not have a battery and in most cases does not have a crystal or other time-base source.        Active Transceiver—A radiating radio tag that actively receives digital data and actively transmits data by providing power to an antenna, and has a battery and in most cases a crystal or other internal time base source.        Inductive Mode—Uses low frequencies, 3-30 kHz VLF or the Myriametric frequency range, 30-300 kHz LF the Kilometric range, with some in the 300-3000 kHz, MF or Hectometric range (usually under 450 kHz). Since the wavelength is so long at these low frequencies over 99% of the radiated energy is magnetic as opposed to a radiated electric field. Antennas are significantly (10 to 1000 times) smaller than the ¼ wave length or 1/10 wave length that would be required to radiate an electrical field efficiently.        Electromagnetic Mode—As opposed to Inductive mode radiation above, uses frequencies above 3000 kHz, the Hectometric range typically 8-900 MHz where the majority of the radiated energy generated or detected may come from the electric field and a ¼ wave or 1/10 length antenna or design is often possible and is used. The majority of radiated and detected energy is an electric field.        
Many of the patents referenced do not make many distinctions outlined in the above glossary and their authors may not at that time been fully informed about the functional significance of the differences outlined above. For example, many of the early issued patents (e.g. U.S. Pat. No. 4,724,427, U.S. Pat. No. 4,857,893, U.S. Pat. No. 3,739,376, U.S. Pat. No. 4,019,181) do not specify the frequency for the preferred embodiment yet it has become clear that dramatic differences occur in performance and functional ability depending on the frequency. The frequency will change the radio tag's ability to operate in harsh environments, near liquids, or conductive materials, as well as the tag's range and power consumption and battery life.
One of the first references to a radio tag in the patent literature was a passive radiating transponder described in U.S. Pat. No. 3,406,391: VEHICLE IDENTIFICATION SYSTEM issued in 1968. The device was designed to track moving vehicles. U.S. Pat. No. 3,406,391 teaches that a carrier signal may be used to communicate to a radio tag as well as provide power. The tags were powered using microwave frequencies and many subcarrier frequencies were transmitted to the tag. The radio tag was programmed to pre-select several of the subcarriers and provided an active re-transmission back when a subcarrier message correspond to particular pre-programmed bits in the tag. This multifrequency approach limited data to about five bits to eight bits and the range of the devices was limited to only a few inches.
U.S. Pat. No. 3,541,257, COMMUNICATION RESPONSE UNIT issued in 1970 further teaches that a digital address may be transmitted and detected to activate a radio tag. The radio tag may be capable of transmitting and receiving electromagnetic signals with memory and the radio tag may work within a full addressable network and has utility in many areas. Many other similar devices were described in the following years (e.g. The Mercury News, RFID pioneers discuss its origins, Sun, Jul. 18, 2004).
U.S. Pat. No. 3,689,885: INDUCTIVELY COUPLED PASSIVE RESPONDER AND INTERROGATOR UNIT HAVING MULTIDIMENSION ELECTROMAGNETIC FIELD CAPABILITIES issued in 1972 and U.S. Pat. No. 3,859,624: INDUCTIVELY COUPLED TRANSMITTER-RESPONDER ARRANGEMENT also issued in 1972 teach that a passive radiating digital radio tag may be powered and activated by induction using low frequencies (50 kHz) and transmit coded data back modulated at higher frequency (450 kHz) to an integrator. It also teaches that the clock and 450 kHz transmitting carrier from the radio tag may be derived from the 50 kHz induction power carrier. The named inventors propose the use of a ceramic filter to multiply the 50 kHz signal nine times to get a frequency regeneration for the 450 kHz data-out signal. These two patents also teach that steel and other conductive metals may detune the antennas and degrade performance. The ceramic filter required to increase the frequency from 50 kHz to a high frequency is, however, an expensive large external component, and phase-locked loops or other methods commonly used to multiply a frequency upward would consume considerable power. These tags use a low frequency “power channel” to power the tag, to serve as the time base for the tag, and finally to serve as the trigger for the tag to transmit its ID. Thus, the power channel contains a single bit of on/off information.
This is shown in FIG. 1. The active low frequency transceiver tag consists of four basic components:                1. the antenna, typically a wound loop or coil, that has been tuned to low frequency (50 kHz)        2. a ceramic filter to multiply the low frequency up to a higher frequency (e.g. 450 kHz)        3. logic and        4. storage meansto generate an active signal that drives an antenna and transmits the tag's ID.        
In contrast, as will be described in detail below, the present invention uses the carrier only as a power source and time-base generator. It does not necessarily use the carrier to trigger the transmission of the ID. This makes it possible for the tag to use half-duplex protocol that makes it possible for the tag to be written and read by an active radiating tag.
U.S. Pat. No. 3,713,148: TRANSPONDER APPARATUS AND SYSTEM issued in 1973 teaches that the carrier to the transponder may also transmit digital data and that the interrogation means (data input) may also be used to power the transponder. This patent also teaches that nonvolatile memory may be added to store data that might be received and to track things like use and costs for tolls. The inventors do not specify or provide details on frequency or antenna configurations.
The devices referenced above all rely on the antenna in radiating transceiver mode, where the power from the radio tag is actually “pumped” into a tuned circuit that includes a radiating antenna, which in turn produces an electromagnetic signal that can be detected at a distance by an interrogator.
U.S. Pat. No. 3,427,614 WIRELESS AND RADIOLESS (NONRADIANT) TELEMETRY SYSTEM FOR MONITORING CONDITIONS issued in 1969 was among the first to teach that the radio tag antenna may communicate simply by detuning the antenna rather than radiating power through the tuned antenna. The change in tuned frequency may be detected by a base-station generating a carrier. This non-radiating mode reduces the power required to operate a tag and puts the detection burden on the base station. In effect the radio tag's antenna becomes part of a tuned circuit created by the combination of the base-station, and a carrier. Any change in the radio tag's tuned frequency by any means can be detected by the base-station's tuned carrier circuit. This is also often referred to as a back-scattered mode and is the basis for most modern RF-ID radio tags.
Many Electronic Article Surveillance (EAS) systems also function using this backscattered non-radiating mode (U.S. Pat. No. 4,774,504 1988, U.S. Pat. No. 3,500,373 1970, U.S. Pat. No. 5,103,234: Electronic article surveillance system, 1992) and most are also inductive frequencies. Many other telemetry systems in widespread use for pacemakers implantable devices, and sensors in rotating centrifuges (U.S. Pat. No. 3,713,124: TEMPERATURE TELEMETERING APPARATUS, 1973) also make use of this backscattered mode to reduce power consumption. U.S. Pat. No. 4,361,153 (Implant telemetry system 1982) teaches that low frequencies (Myriametric) can transmit though conductive materials and work in harsh environments. Most of these implantable devices also use backscattered communication mode for communication to conserve battery power.
Thus, more recent and modern RF-ID tags are passive, backscattered transponder tags and have an antenna consisting of a wire coil or an antenna coil etched or silk-screened onto a PC board (e.g. see U.S. Pat. No. 4,857,893: Single chip transponder device, 1989; U.S. Pat. No. 5,682,143: Radio frequency identification tag, 1997). These tags use a carrier that is reflected back from the tag. The carrier is used by the tag for four functions:                1. The carrier contains the incoming digital data stream signal; in many cases the carrier only performs the logical function to turn the tag on/off and to activate the transmission of its ID. In other cases the data may be a digital instruction.        2. The carrier serves as the tag's power source. The tag receives a carrier signal from a base station and uses the rectified carrier signal to provide power to the integrated circuitry and logic on the tag.        3. The carrier serves as a clock and time base to drive the logic and circuitry within the integrated circuit. In some cases the carrier signal is divided to produce a lower clock speed.        4. The carrier may also in some cases serve as a frequency and phase reference for radio communications and signal processing. The tag can use one coil to receive a carrier at a precise frequency and phase reference for the circuitry within the radio tag for communications back through a second coil to the reader/writer making accurate signal processing possible. (U.S. Pat. No. 4,879,756 Radio broadcast communication systems, 1989).        
Thus the main advantage of a passive backscattered transponder is that it eliminates the battery as well as a crystal in LF tags. HF and UHF tags are unable to use the carrier as a time base because the speed would require high speed chips and power consumption would be too high. It is therefore generally assumed that a passive backscattered transponder tag is less costly than an active or transceiver tag since it has fewer components and is less complex.
These modern non-radiating, transponder backscattered RFID tags typically operate at frequencies within the Part 15 rules of the FCC (Federal Communication Commission) between 10 kHz to 500 kHz (Low Frequency or Ultra Low Frequency ULF), 13.56 MHz (High Frequency, HF) in or 433 MHz (MHF) and 868/915 MHz or 2.2 GHz (Ultra High Frequency UHF). The higher frequencies are typically chosen because they provide high bandwidth for communications, on a high-speed conveyor for example, or where many thousands of tags must be read rapidly. In addition, it is generally believed that the higher frequencies are more efficient for transmission of signals and require much smaller antennas for optimal transmission. (It may be noted that a self-resonated antenna for 915 MHz can have a diameter as small as 0.5 cm and may have a range of tens of feet.)
The major disadvantage of the backscattered mode radio tag, however, is that it has limited power, limited range, and is susceptible to noise and reflections over a radiating active device. This is not because of loss of communication signal but instead is largely because the passive tag requires a minimum of 1 volt on its antenna to power the chip. As a result many backscattered tags do not work reliably in harsh environments and require a directional “line of sight” antenna.
One proposed method to extend the range of a passive backscattered tag has been to add a thin flat battery to the battery to the backscattered tag so that the power drop on the antenna is not the critical range limiting factor. However, since all of these tags use high frequencies the tags must continue to operate in backscattered mode to conserve battery life. The power consumed by any electronic circuit tends to increase with the frequency of operation. Thus, if a chip were to use an industry standard 280 mAh capacity CR2525 Li cell (which is the size of a quarter) we would expect battery life based solely on operating frequency to be:
Assumes 280 mAh Li BatteryPower (uAHr)PredictedFreq.Current (uA)LifeUnits128kHz131.00Years13.56MHz1023.78Months915MHz7,0311.66Days
Thus most recent active RFID tags that may have a battery to power the tag circuitry, such as active tags and devices operating in the 13.56 MHz to 2.3 GHz frequency range, also work as backscattered transponders (U.S. Pat. No. 6,700,491: Radio frequency identification tag with thin-film battery for antenna, 2004; also see US20040217865A1: RFID tag 2004, for detailed overview of issues). Because these tags are active backscattered transponders they cannot work in an on-demand peer-to-peer network setting, and they require line-of-sight antennas that provide a carrier that “illuminates” an area or zone or an array of carrier beacons.
Active radiating transceiver tags in the high-frequency range (433 MHz) that can provide on-demand peer-to-peer network of tags are available (e.g. SaviTag ST-654, U.S. Pat. No. 5,485,166: Efficient electrically small loop antenna with a planar base element, 1996) and full visibility systems described above (U.S. Pat. No. 5,686,902, U.S. Pat. No. 6,900,731). These tags do provide full functionality and what might be called Real-Time Visibility, but they are expensive (over $100.00 US) and large (videotape size, 6¼×2⅛×1⅛ inches) because of the power issues described above and must use replaceable batteries since even with such a 1.5 inch by 6 inch Li battery these tags are only capable of 2,500 reads and writes.
It is also generally assumed that a an HF or UHF passive backscattered transponder radio tags will have a lower cost-to-manufacture as compared with an LF passive backscattered transponder because of the antenna. An HF or UHF tag can obtain a high-Q 1/10-wavelength antenna by etching or use of conductive silver silk-screening the antenna geometry onto a flexi circuit. An LF or ULF antenna cannot use either because the Q will be too low due to high resistance of the traces or silver paste. So LF and ULF tags must use wound coils made of copper.
Thus, in summary a passive transponder tag has the potential to lower cost by eliminating the need for a battery as well as an internal frequency reference means. An active backscattered transponder tag eliminates the extra cost of a crystal but also provides for enhanced amplification of signals over a passive backscattered transponder and enhanced range. In addition, it is also possible to use a carrier reference to provide enhanced anti-collision methods so as to make it possible to read many tags within a carrier field (U.S. Pat. No. 6,297,734, U.S. Pat. No. 6,566,997, U.S. Pat. No. 5,995,019, U.S. Pat. No. 5,591,951). Finally active radiating transceiver tags require large batteries, are expensive and may cost tens to hundreds of dollars.
A second major area of importance to this invention is the use of two co-planer antennas in radio tags placed in such a way as to inductively decouple the antennas from each other so they may be independently tuned. U.S. Pat. No. 2,779,908: Means for reducing Electro-Magnetic Coupling 1957 teaches that electromagnetic coupling of two co-planer air-core coils may be minimized by shifting the coils as well as placing a neutralizing shorted coil inside the area of the two coils. U.S. Pat. No. 4,922,261: (Aerial systems, 1990) teaches that this may be used in a passive transponder tag in that two frequencies and two antennas may be used, one for transmitting data and a second for receiving data thereby providing double the communication speed with full-duplex data transfers. U.S. Pat. No. 5,012,236: (Electromagnetic energy transmission and detection apparatus, 1991) makes use of decoupled coils to enhance range and minimize sensitivity to angles. FIG. 2 shows the arrangement and method to decouple two antennas described by U.S. Pat. No. 4,922,261. In this case one antenna is used for transmitting data, and the second is used for receiving data. The antenna arrangement makes it possible to have two data communication frequencies so the tag can communicate with a full-duplex protocol.
U.S. Pat. No. 6,584,301: Inductive reader device and method with integrated antenna and signal coupler, 2003) also discloses a co-planer geometry that minimizes coupling between two coils. The purpose was to enable a two-frequency full-duplex mode of communication to enhance communications speed. In most cases the speed of communication is not a critical issue in visibility systems and other applications described below. FIG. 3 shows this coil arrangement to decouple two antennas. Coil 6 is shifted in the same plane from coil 5. The primary purpose disclosed in the prior art is to provide higher data communication speeds between tag and the base station.
U.S. Pat. No. 6,176,433: (Reader/writer having coil arrangements to restrain electromagnetic field intensity at a distance, 2001) makes use of a co-planer coil to enhance range of a backscattered transponder tag used as a IC card and using a 13.56 MHz carrier. The isolated antennas may be used to communicate to the tag and to maximize power required to transmit to the tag under within the limits of the Wireless Communications Act.
Many publications and patents teach the advantages of using RFID tags for tracking products in warehouses, packages etc. In some cases passive transponders may be used but additional location and automated systems may be required for the base-station (e.g. U.S. Pat. No. 6,705,522: Mobile object tracker, 2004). However, most investigators now recognize that a fully integrated peer-to-peer on-demand network approach using active radio tags has many functional advantages in these systems over a system (U.S. Pat. No. 6,705,522: Mobile object tracker, 2004; U.S. Pat. No. 6,738,628: Electronic physical asset tracking, 2004; US20020111819A1: Supply chain visibility for real-time tracking of goods, 2002; U.S. Pat. No. 6,900,731: Method for monitoring and tracking objects, 2005; U.S. Pat. No. 5,686,902: Communication system for communicating with tags, 1997; U.S. Pat. No. 4,807,140: Electronic label information exchange system, 1989). One of the major disadvantages of a passive nonradiating system is that it requires the use of handheld readers or portals to read tags and changes in process control (e.g. U.S. Pat. No. 6,738,628: Electronic physical asset tracking, 2004). A system that provides data without process change and without need to carry out portal reads is more likely to be successful as a visibility system.
It will also be appreciated that the prior art has assumed low frequency tags to be slow, short range, and too costly. For example, both U.S. Pat. No. 5,012,236, U.S. Pat. No. 5,686,902 discuss the short-range issues associated with magnetic induction and low frequency tags. Because of the supposed many apparent disadvantages of ULF and LF, the RF-ID frequencies now recommended by many commercial (Item-Level Visibility In the Pharmaceutical Supply Chain: A Comparison of HF, UHF RFID Technologies, July 2004, Texas Instruments, Phillips Semiconductors, and TagSys Inc.), government organizations (see Radio Frequency Identification Feasibility Studies and Pilot, FDA Compliance Policy HFC-230, Sec 400. 210, November, 2004, recommend use of LF, HF or UHF) as well as standards associations (EPCglobal, web page tag specifications, January 2005, note LF and ULF are excluded) do not mention or discuss the use of ULF as an option in many important retail applications. Many of the commercial organizations recommending these higher frequencies believe that passive and active radio tags in these low frequencies are not suitable for any of these applications for reasons given above.
In addition, several commercial companies actually manufacture both ULF and LF radio tags (e.g. both Texas Instruments and Philips Semiconductor. See Item-Level Visibility In the Pharmaceutical Supply Chain: A Comparison of HF, UHF RFID Technologies, July 2004, Texas Instruments, Phillips Semiconductors, and TagSys Inc.) yet only recommend the use of 13. 56 MHz or higher again because of the perceived disadvantage of ULF and LF outlined above, and the many perceived advantages of HF and UHF).
In sum, system designers for modern applications have chosen not to use LF radio tags because:                1. ULF is believed to have very short range since it uses largely inductive or magnetic radiance that drops off proportional to 1/d3 while far-field HF and UHF drops off proportional to 1/d, where d is distance from the source. Thus, the inductive or magnetic radiance mode of transmission will theoretically limit the distance of transmission, and that has been one of the major justifications for use of HF and UHF passive radio tags in many applications.        2. The transmission speed is inherently slow using ULF as compared to HF and UHF since the tag must communicate with low baud rates because of the low transmission carrier frequency.        3. Many sources of noise exist at these ULF frequencies from electronic devices, motors, fluorescent ballasts, computer systems, power cables.        4. Thus ULF is often thought to be inherently more susceptible to noise.        5. Radio tags in this frequency range are thought to be more expensive since they require a wound coil antenna because of the requirement for many turns to achieve optimal electrical properties (maximum Q). In contrast HF and UHF tags can use antennas etched directly on a printed circuit board and ULF would have even more serious distance limitations with such an antenna.        6. Current networking methods used by high frequency tags, as used in HF and UHF, are impractical due to such low bandwidth of ULF tags described above in point 3 immediately above.        
It should be appreciated that the above-mentioned RF tags are antithetical to an “area read”. With the above-mentioned RF tags, whenever the tag is powered, it immediately transmits its message. If the tag is powered again, it transmits its message again. If several RF tags are nearby to each other, then if they are powered, they all transmit their respective messages. This collision-prone circumstance repeats itself every time the RF tags are powered. It would be very desirable to have a system in which the RF tags were to respond in a way that facilitates “area reads”.